The present application is based on Japanese Patent Application No. 2001-294956 filed on Sep. 26, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a flexible wiring board used in electronic apparatus such as a portable terminal apparatus, and a method for producing the flexible wiring board, particularly, a method for forming a via hole in the flexible wiring board.
2. Description of the Related Art
The mobile and wearable demand for portable terminal apparatus such as cellular phones has increased with the remarkable advance of technological innovation in recent years. Flexible wiring boards have been required to be finely patterned and densely provided with the advance of reduction in weight and thickness and increase in density of the electronic apparatus.
Reduction in size and pitch of electric connection portions is required for patterning a flexible wiring board finely. Formation of three-dimensional wiring in a flexible wiring board is also required for increasing density. That is, because the flexible wiring board is a laminate of layers constituted by wiring boards having desired circuit patterns, the flexible wiring board needs to have a structure in which the layers are electrically connected to one another through via holes subjected to a plating process. Therefore, reduction in pitch and size of the via holes used for such electric connection is required for achieving both fine patterning and increase in density simultaneously.
When both pitch and size of the via holes are reduced as described above, reliability of electric connection after the plating process is largely influenced by the quality of the processed via holes. However, if the via holes are formed by a drill, burrs are generated. If the via holes are formed by an infrared laser beam, the electric conductor layer does not absorb the infrared laser beam so that the electrically insulating layer is damaged by heat accumulated in the via holes. Hence, greater reduction in size of the via holes can be hardly achieved by use of a drill or an infrared laser beam because of the problems in projections on the electric conductor layer and damage of the electrically insulating layer.
Therefore, a method of forming a via hole by an ultraviolet laser beam has been used in recent years, so that the formation of fine-size via holes has been realized. It is however impossible to perfectly solve the problems in burrs or projections and deposition of smears even in the case where an ultraviolet laser beam is used. To achieve greater reduction in size of the via holes, a proposal for a forming method to reduce these problems is desired.
In consideration of such circumstances, an object of the invention is to propose a method for forming a via hole substantially free from these problems to thereby make it possible to achieve reduction in pitch and size of the via hole and to provide a flexible wiring board which meets the requirements of finer patterning and increase in density.
The present inventors have made researches into solution of the problems while paying attention to energy density of an ultraviolet laser beam applied on a via hole-forming portion. As a result, the invention having the following features has been already accomplished.
(1) A method of forming a via hole in a flexible wiring board having at least two electric conductor layers and at least one electrically insulating layer for insulating said two electric conductor layers from each other, said method comprising:
irradiating one of said electric conductor layers with a first ultraviolet laser beam; and
irradiating said electrically insulating layer with a second ultraviolet laser beam;
wherein energy density of said first ultraviolet laser beam is higher than energy density of said second ultraviolet laser beam.
(2) A method of forming a via hole in a flexible wiring board according to (1), wherein energy density of said first ultraviolet laser beam is selected to be higher by a value ranged from 0.1 to 20 J/cm2 than energy density of said second ultraviolet laser beam.
(3) A method of forming a via hole in a flexible wiring board according to (1) or (2), wherein energy density of said first ultraviolet laser beam is selected to be in a range of from 5 to 20 J/cm2, whereas energy density of said second ultraviolet laser beam is selected to be in a range of from 0.05 to 5 J/cm2.
(4) A method of forming a via hole in a flexible wiring board according to (1) through (3), said method comprising the steps of:
irradiating a via hole-forming portion of said one of said electric conductor layers with said first ultraviolet laser beam to thereby form an opening in said one electric conductor layer; and
subsequently irradiating said electrically insulating layer with said second ultraviolet laser beam to thereby form an opening in said electrically insulating layer after forming the opening in said one electric conductor.
(5) A method of forming a via hole in a flexible wiring board by repeating the steps defined in (4).
(6) A method of forming a via hole in a flexible wiring board according to (1) through (3), said method comprising the steps of:
irradiating a via hole-forming portion of a first layer of said electric conductor layers with said first ultraviolet laser beam to thereby form an opening in said first electric conductor layer; and
irradiating said electrically insulating layer with said second ultraviolet laser beam to thereby form an opening in said electrically insulating layer; and
irradiating a second layer of said electric conductor layers which is located on opposite side to said first electric conductor layer with respect to said electrically insulating layer with said first ultraviolet laser beam to thereby form an opening in said second electric conductor layer, whereby said openings are formed as the via hole in said flexible wiring board.
(7) A method of producing a flexible wiring board, including a method defined in any one of (1) through (6).
(8) A flexible wiring board produced by a method defined in (7).